The invention relates to a method and apparatus for controlling heat bonding, particularly but not exclusively for controlling heat bonding carried out by means of heat presses for use in, for example, the heat sealing application of badges, labels, transfers etc. to fabric.
Most processes which involve the fusing or curing of products by the use of heat, such as the heat-sealing application of badges, labels or transfers to fabrics or other materials, the lamination of, for example, two substrates via hot melt adhesives, or the curing of materials, such as, for example, rubber, are achieved at present through the controlled use of heat within a prescribed time.
Existing heat bonding or sealing machinery uses a dwell timing device to control the sealing/transfer cycle. Heat is applied by conduction from a pre-heated platen being placed on top of the item to be sealed, transfer printed or otherwise bonded. The products used in heat bonding or sealing typically incorporate a heat-activated xe2x80x9cadhesivexe2x80x9d coating (or laminate) or the product itself may incorporate a heat activated ingredient which allows it to be transferred from a carrier to a fabric or other material. Depending upon the composition of the heat seal product (and to some extent the material to which it is to be affixed) a temperature has to be achieved which is sufficient to activate the adhesive element and so effect a bond to the material.
Manufacturers of such heat bonding and heat sealable products provide recommendations in respect of temperature/time and pressure which take into account some of the shortcomings of the equipment. Thus, a recommendation of 200xc2x0 C. for 10 seconds provides only approximate conditions at the interface of product and fabric since various factors (such as the temperature or moisture content of the fabric and the temperature of the base platen) will affect the temperature actually achieved at the interface within any timescale less than that required for the top platen to bring the fabric and base up to its operating temperature. The recommended time given assumes that enough heat is transferred from the plate to melt the adhesive, which results in products being submitted to wide temperature variances during application, which is not an optimum situation. In early runs with the machine, heat will be transferred to the surroundings, which may result in insufficient heat being applied to the product. Alternatively, in later runs, the time given may be in excess of that needed for optimum operation, resulting in overheating of the heat-bonding product or inefficient use of the machinery. The problem can be reduced by heating both plates but this adds to the cost without completely solving the problem.
GB patent no. 1,265,009 discloses a textile pressing machine in which the bonding of sheets of fabric is controlled by a timer. The device further includes a surface temperature sensor positioned on the unheated face of the press and, after a pre-set time for the process has been completed, the temperature is sensed and if it has not reached a pre-set temperature then the process is continued until the temperature is reached.
However, this device does not solve the problem of potential excess process time over the optimum.
Additionally, in order to control the temperature to which a product is exposed during the heat sealing process, it is necessary to measure the temperature as close to the xe2x80x9cglue linexe2x80x9d or bonding area as possible. Known presses used for garment decoration purposes have a heated top platen or may have both platens heated and known temperature sensors are unsuitable for mounting on a heated platen as they measure a preset temperature. Such temperature sensing devices are, therefore, inappropriate for fitting to existing heat presses and thus cannot be retrofitted to existing equipment.
A further problem with prior art devices in which the temperature is measured is that they rely on accurate temperature calibration of the thermostatic control of the press which is directly linked to the preset temperature. If the calibration is inaccurate, the temperature attained during the process will also be inaccurate.
It is an object of the present invention to provide more accurate control of the process of a heat bonding apparatus.
The present invention provides an apparatus for controlling a heat bonding process in a heat bonding apparatus comprising pressing means for applying pressure to an article in a heat bonding cycle and heating means to provide heat to the article during the heat bonding cycle, comprising:
means for sensing the temperature at an interface between the pressing means and the article to be pressed; and
signal processing means; wherein the means for sensing the temperature at the interface provides signals corresponding to the temperature at the interface to the signal processing means and the signal processing means detects a rapid change in temperature at the interface and provides a start-of-cycle signal indicating initiation of the heat bonding cycle in response to said rapid change in temperature at the interface and, following provision of the start-of-cycle signal, the signal processing means detects the attainment of a pre-set temperature reached at the interface and provides an end-of-cycle signal signifying the completion of the heat bonding cycle in response to said attainment of said pre-set temperature reached at the interface.
The rapid change in temperature detected by the signal processing means indicates the start of the heat bonding cycle. Following the start of the heat bonding cycle, the temperature at the interface rises until it reaches the final pre-set temperature at which the end-of-cycle signal is provided. The signal processing means only provides an end-of-cycle signal when the pre-set temperature is reached following a start-of-cycle signal. This enables the heat bonding cycle to be accurately controlled in that the cycle only starts following the detection of a rapid change in temperature and is not dependent on an operator signalling the start of a cycle.
Preferably the means for sensing the temperature at the interface comprises a temperature sensing device that is small enough so that it does not interfere with the heat bonding process. Preferably the means for sensing the temperature at the interface comprises a temperature sensing device of low thermal mass so as to be capable of detecting very rapid changes of temperature. An example of a suitable temperature sensor is a thermocouple device but any suitable sensor may be used.
The heat bonding apparatus may comprise a heat seal press, a heat fusion machine, apparatus for transfer printing and fusion, apparatus for hot melt printing, thermal fixing apparatus, apparatus for the lamination of, for example, two substrates via hot melt adhesives in which the substrates are held in position by means of clamps or frames, and moulds for the curing of materials, such as, for example, rubber.
In a preferred embodiment, the invention provides an apparatus for controlling a heat bonding process in a heat seal press comprising two platens for applying pressure to an article in a heat bonding cycle and heating means to heat at least one of the platens to provide heat to the article during the heat bonding cycle. The apparatus preferably comprises a temperature sensor arranged between the platens and the signal processing means comprises a sensor processing unit. Advantageously the temperature sensor is attached to the inward facing side of a heated, higher temperature, platen and the sensor processing unit responds to a rapid drop in temperature to provide a signal to indicate the start of the heat bonding cycle.
Alternatively the temperature sensor may be attached to the inward facing side of the unheated or lower temperature platen or arranged between the platens, in which cases the sensor processing unit responds to a rapid rise in temperature to provide a signal to indicate the start of the heat bonding cycle.
When the temperature sensor reaches a pre-set second temperature, the sensor processing unit provides an end-of-cycle signal signifying the completion of the heat bonding cycle in response to said second signal.
Advantageously, for a manually operated heat bonding apparatus, the sensor processing unit may provide an audible or visual alarm to indicate to an operator that the heat bonding cycle is complete, or, for an automatic heat bonding apparatus, the sensor processing unit may provide an end-of-cycle signal to lift the platen automatically.
Advantageously the pre-set temperature can be set depending on the materials used.
Advantageously, for a manually operated heat bonding apparatus, when the sensor processing unit detects a further rapid change in temperature after the end-of-cycle signal, a signal is produced to switch off the alarm. The next rapid change in temperature then indicates the start of the next cycle.
The apparatus according to the invention may comprise an integral part of the control mechanism of a heat bonding apparatus or may comprise a stand alone device which can be fitted to a heat bonding apparatus.
The invention further provides a method of controlling a heat bonding process in a heat bonding apparatus comprising pressing means for applying pressure to an article in a heat bonding cycle and heating means to provide heat to the article during the heat bonding cycle, comprising the steps of:
sensing the temperature at an interface between the pressing means and the article to be pressed;
providing a first signal corresponding to a rapid change in temperature at the interface;
providing a start-of-cycle signal signifying initiation of the heat bonding cycle in response to said first signal;
following provision of the start-of-cycle signal, providing a second signal corresponding to a pre-set temperature reached at the interface; and
providing an end-of-cycle signal signifying the completion of the heat bonding cycle in response to said second signal.
Preferably the method includes the step of providing an alarm in response to the end-of-cycle signal. For a manually operated machine, this may be an audible or visible alarm to indicate to an operator that the cycle is complete. Alternatively, for an automatically operated machine, the method may include the step of providing a signal to the operating mechanism of an automatically operated machine to automatically end the cycle.
Advantageously, for a manually operated machine, the method includes the step of detecting a further rapid change in temperature after the end-of-cycle signal as a result of the retraction of the heating and pressing means and providing a signal in response to the further rapid change to switch off the alarm.
The present invention controls the process cycle by reference to the actual temperature at the interface. This is achieved by placing a temperature sensor linked to an electronic alarm device which alarms at the achievement of a pre-set desired temperature. The alarm may be an audible buzzer or flashing light (in the case of a manually operated machine) or a signal sent to the operating mechanism in the case of an automatically operated machine.
The present invention measures the actual temperature of the interplaten margin and predicts the successful application of the heat bonded product in terms of the actual inter-platen temperature attained, rather than, as is the case presently, through the time that the process should take, since this time (the dwell time) varies significantly as a result of factors which are outside the effective control of existing heat presses and operators of such presses.
The device of the present invention monitors the temperature of the interplaten margin independently of the thermostatic control of the platen and thus overcomes any potential problems that may be associated with inaccurate calibration of the machine by providing a known rather than an assumed temperature exposure for the article in the press. The device of the present invention has the added advantage that it can be used to check the calibration of a pressing machine.
Factors such as ambient temperature, product temperature, fabric moisture content, heated platen temperature and the frequency of operation will affect the dwell time necessary to expose the product to a pre-set temperature.